1. Field of the Invention
The present invention relates to a fuel injection control apparatus of an internal combustion engine which controls the amount of fuel injection in accordance with control laws set based on a fuel dynamic model which describes the dynamics of fuel flowing into the cylinder of the internal combustion engine.
2. Description of the Related Art
In the past, there has been known, as an apparatus for controlling the amount of fuel injection so that the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine becomes a target air-fuel ratio, for example, the control apparatus based on the so-called linear control theory which, as described in Japanese Unexamined Patent Publication (Kokai) No. 59-196930, uses as control input a correction value for correcting a basic fuel injection amount found from the rotational speed and amount of intake air of the internal combustion engine and uses as control output the actual measured value of the air-fuel ratio detected using an air-fuel ratio sensor so as to identify that a linear approximation stands between the control input and control output, finds the mathematical model describing the dynamic characteristics of the internal combustion engine, and controls the fuel injection amount by control laws based on the same.
The relationship between the control input and the control output, however, is inherently nonlinear and it is only possible to describe the dynamic characteristics of an internal combustion engine under extremely narrow operating conditions, since the mathematical model is found using just linear approximation, as mentioned above. To achieve excellent control, it is necessary, as described in Japanese Unexamined Patent Publication (Kokai) No. 59-7751, to set a mathematical model for each of a plurality of operating regions where it is possible to deem that linear approximation stands and, based on this, to determine the control laws for each of the operating regions.
Therefore, in the above conventional apparatus, there has been the problem that it is necessary to change the control laws for each of the operating regions of the internal combustion engine, making the control complicated. Further, there was the problem of unstable control due to the switching of the control laws at the boundaries of the operating regions.
To resolve this problem, proposal was made of a fuel injection control apparatus which could execute fuel injection control without switching control laws as mentioned above by determining nonlinear compensation control laws based on a fuel dynamic model describing the dynamics of the fuel in the internal combustion engine (that is, using a single set of control laws) (see U.S. Pat. No. 4,903,668).
However, it is difficult to accurately describe at all times the dynamics of fuel in an internal combustion engine even with the above-mentioned fuel dynamics. In actuality, the fuel dynamic model loses its correspondence with the actual fuel dynamics due to changes in the engine characteristics along with time and if the control laws of the time of setting are used as they are for execution of fuel injection control, it sometimes becomes impossible to ensure the control accuracy of the air-fuel ratio. In particular, when the internal combustion engine is operated for a long period, deposits form on the walls of the intake pipe, whereby the model parameter expressing the proportion of the fuel sticking to the wall of the intake pipe in the above-mentioned fuel dynamic model becomes different from reality and as a result the control accuracy of the air-fuel ratio ends up reduced.
To resolve this problem, it has been considered to calculate the model parameter based on the amount of fuel supplied by injection to the internal combustion engine, the amount of air flowing into the cylinders, and the air-fuel ratio detected based on the components of the exhaust of the internal combustion engine and to correct the control laws based on the results of the calculation. The proposed apparatus finds the time intervals of the change of the air-fuel ratio from lean to rich or rich to lean based on the above-mentioned amount of fuel, amount of air, and air-fuel ratio and estimates the model parameter based on this time interval so as to enable correction of the control error along with changes in the above model parameter. According to this apparatus, automatic control is performed so as to make the control laws correspond to the actual fuel dynamics, making it possible to improve the control accuracy of the air-fuel ratio.
Examining this apparatus in more detail, if the fuel injected into the internal combustion engine changes in fuel properties to something different from the time of design, an error occurs with respect to the estimated value of the model parameter and it becomes impossible to correct the control laws well, it was learned.
That is, first, when the characteristics of the fuel injected into the internal combustion engine change, there is a change not only in the proportion of the injected fuel sticking to the walls of the intake pipes, but also the amount of the fuel sticking to the walls of the intake pipe directly flowing into the cylinder. Therefore, the time interval from the change of the air-fuel ratio, calculated based on the amount of fuel amount of air, and air-fuel ratio, from lean to rich or rich to lean changes according also to the amount of change of the amount of fuel sticking to the walls of the intake pipe flowing directly into the cylinder due to the changes in the characteristics of the fuel injected. If, as mentioned above, the model parameter expressing the proportion of injected fuel sticking to the walls of the intake pipe is continuously calculated, the model parameter will be erroneously estimated and it will end up impossible to correct the control laws well.